The present invention relates to an apparatus and method for storing, mixing and/or dispensing a fluid. More specifically, embodiments of the invention relate to an apparatus and method for storing, mixing and/or dispensing fluid to a substrate processing system. Embodiments of the invention are particularly useful for storing and dispensing spin-on-dielectric (SOD) formulations but may also be used to store and dispense other fluid formulations, such as chemical mechanical polishing (CMP) solutions and others.
Semiconductor device geometries have dramatically decreased in size since integrated circuits were first introduced several decades ago, and all indications are that this trend will continue. Although today's wafer fabrication plants are routinely producing ever-shrinking devices, the plants of the future will soon be producing devices having even smaller geometries.
In order to continue to reduce the size of devices on integrated circuits, it has become necessary to use insulators having low dielectric constants. Such films are particularly desirable for premetal dielectric (PMD) layers and intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metalization, to prevent crosstalk between the different levels of metalization, to reduce signal reflection, and to reduce device power consumption. To this end, several semiconductor manufacturers, materials suppliers, and research organizations have focused on identifying low-dielectric-constant films. As used herein, low-dielectric-constant (low-k) films are those having a dielectric constant below 3.0 including films having a dielectric constant below 2.0.
Some approaches to developing such low-k films include introducing porosity into known dielectric materials to reduce the material's dielectric constant. Dielectric films when made porous, tend to have lower dielectric constants (the dielectric constant of air is normally 1.0). One particular class of porous low-k films includes mesoporous silica materials. One known method of forming such mesoporous silica films is referred to as the sol gel process, in which high porosity films are produced by hydrolysis and polycondensation of a metal oxide.
The sol gel process is a versatile solution process for making ceramic material. In general, the sol gel process involves the transition of a system from a liquid “sol” (mostly colloidal) into a solid “gel” phase. The starting materials used in the preparation of the sol are usually inorganic metal salts or metal organic compounds such as metal alkoxides. The precursor solutions are typically deposited on a substrate by spin on methods. In a typical sol gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a sol. Further processing of the sol enables one to make ceramic materials in different forms. One method of forming such mesoporous low-k films is described in U.S. application Ser. No. 09/823,932, filed on Mar. 29, 2001 in the name of Robert P. Mandel et al. and assigned to Applied Materials, Inc., the assignee of the present case. The Ser. No. 09/823,932 application is hereby incorporated by reference in its entirety.
Some low-k SOD formulations have a variety of constituent colloids with highly variant densities. In the sol-gel phase, these variant densities cause the constituent colloids of the SOD formulation to stratify. Dispensing stratified SOD formulations may result in the formation low-k films having spatially varying irregularities, such as varying film thickness, refractive index, and dielectric constant among other undesirable properties. Vessels have been developed for mixing low-k SOD formulations to relatively homogeneous consistency prior to dispensing. Such vessels typically mix the SOD formulations by mechanical agitation. One typical agitation mechanism includes, a set of mixing blades inserted in a vessel. The blades are rotated or plunged up and down to mix the SOD formulation and reduce stratification. Other typical agitation mechanisms include a vibration mechanism attached to the vessel that shakes the vessel and hence mixes the SOD formulations contained therein.
Mechanical mixing vessels have been used with some success to mix and dispense low-k SOD formulations in semiconductor fabrication facilities. Such vessels are not without shortcomings, however. For example, during a mechanical mixing process, such as a vibration mixing process, colloid in the low-k SOD formulation is washed onto the walls of the mixing vessel where the colloid condenses to form particles. Continued agitation causes these particles to shed from the walls of the mixing vessel back into the SOD formulation. Upon being mixed back into the SOD formulation, the particles do not dissolve into their constituent parts, but remain as particles, forming impurities in the SOD formulation. The result of dispensing these particles with the low-k SOD formulation is the formation of low-k films having spatially varying irregularities that may adversely affect semiconductor devices. Agitation mechanisms have other shortcomings, such as shedding of particles of the vessel itself into the low-k SOD formulation which may also cause spatially varying irregularities in formed low-k films.
In addition to having a tendency to stratify, some low-k SOD formulations have highly reactive chemistries that have inherently short shelf lives at room temperature (21° C.). At room temperature, some low-k SOD formulations degrade within 24-48 hours. For example, MesoELk™ SOD formulation, a silicon oxide low-k film precursor manufactured by Schumacher, a unit of Air Products and Chemicals, Inc., chemically degrades within approximately 24 hours at room temperature. However, at about −10° C. MesoELk™ SOD formulation can be kept chemically stable for greater than 30 days. Similar to dispensing a stratified or nonuniformly mixed low-k SOD formulation, dispensing chemically degraded SOD formulations yield dielectric films having spatially varying irregularities, such as varying film thicknesses, refractive indexes, and dielectric constants among other undesirable properties.
Accordingly, there is a need for new techniques and mechanisms for storing, mixing, and dispensing solutions, such as SOD formulations, to substrate processing systems.